This invention relates to a method for producing rails by hot-rolling an elongated workpiece in a rolling mill train, having at least two rolling mill stands, each with two roll-pass openings in a pair of horizontal rolls and a vertical roll for each roll-pass opening. More particularly, the present invention relates to such a method for producing rails in which an unused roll-pass opening at one location in the rolling mill train is used at a different location in the rolling mill train by repositioning at least the horizontal rolls, preferably the entire rolling mill stand, at a different location in a manner such that the bottom of a rail product is rolled at one location by a vertical roll and the top of a rail product at a different location by a vertical roll.
In the past, rails were formed by two general methods of rolling. One method was known as a tongue-in-groove, flat or slab and edging, and the second method was known as diagonal or angular. Combinations of the two methods have been used. The former method has been characterized by the axis of symmetry of the rail coinciding with the pitch line and parallel to the train line of the rolls. In the latter method, the rail was shaped in the first pass of the roughing stand instead of first compressing the workpiece to a smaller size and then forming the section partly through compression and partly through spreading. The process of the second method is one of compressing the workpiece from the beginning to the end. The stages of reducing the workpiece, usually identified as a bloom, include a roughing stage where the bloom undergoes a large amount of working, mainly to reduce the size of the section and elongate the bloom. In an intermediate stage, the cross-sectional shape of the bloom is altered to the form of a rail through a combination of slabbing, forming, edging and a leader pass, depending upon the mill layout. In the finishing stage, a finishing pass completes the formation of the rail.
A so-called Gary method for producing rails involves the use of universal rolling mill stands for the leader and finishing passes. In the leader pass, the workpiece is conducted through a pass opening in a pair of horizontal rolls while a head wheel in the form of a vertical roll contacts the top of the rail. In the finishing pass, the workpiece is conducted through a pass opening in a pair of horizontal rolls while a base wheel, in the form of a vertical roll, engages the bottom of the rail. The mill stand for the leader pass may have two leader roll-pass openings, one at each of the opposite sides of the horizontal rolls with a head wheel associated with each leader pass. In a similar way, the finishing stand may have two finishing passes in the horizontal rolls with a base wheel associated with each finishing pass. The so-called Gary method is described in a paper entitled "Operation and Roll Design of the Gary Roll Mill" by R. W. Dickson, published in the Iron and Steel Engineer, January 1953.
In U.S. Pat. No. 3,342,053, there is described a method of producing a rail section in a train of rolling mill stands that includes a universal finishing mill mounted on a turret so that two roll-pass openings can be used by rotating the entire mill stand 180.degree. to reposition an unused roll-pass opening for continued use without changing the rolls. Frequent replacement of the rolls in a mill stand is costly, not only from the standpoint of lost production, but also from the standpoint of the costs for refurbishing and supplying new rolls. To obviate this problem in the past, attempts were made to improve the roll-changing procedure and use one and then the other of the two roll-pass openings when a head wheel or base wheel is used to form part of a roll-pass opening. Roll changing is usually more complicated because at least three, instead of two, rolls must be removed and replaced. It is extremely inefficient and undesirable to substitute a worn roll-pass opening at one end of a pair of rolls for a second and replacement roll-pass opening in the same pair of horizontal rolls. In order to use two roll-pass openings in the same set of rolls at the same location in the mill train after one roll-pass opening is worn, all workpieces must be rotated 180.degree. about their longitudinal axes to orientate the top and bottom surfaces of the rail in the proper relation to the vertical roll of the unused roll gap to continue the rolling operation. Handling of the heated workpieces to meet the requirement for rotation at the final stages of the rolling operation increases the likelihood of damage including bending of the rail flanges. The time needed to rotate the workpieces permits unwanted cooling. If the entire mill stand is rotated 180.degree., then two mill drives must be used, one at each side of the mill. This will overcome the need for rotating the workpiece, but because the rolls can be driven from only one side of the mill, the initial cost for two mill drives is unacceptable. This is because present-day mills provide a thrust bearing on the journal of each horizontal roll at the operator's side to transfer forces acting in an axial direction to the mill housings. Typically, the thrust bearing is mounted on a smaller diameter journal seat surface than the diameter of the journal surface for the main bearings that support the rolls under the rolling force. This "necking-down" of the horizontal roll journals at the operator's side of the mill usually weakens the roll journals to such an extent that they cannot be driven from the ends of the rolls where thrust bearings are used.
Replacing horizontal rolls having a single roll pass in leader and finishing stands, for example, requires an excessively large roll inventory. The additional cost to disassemble chocks from a worn pair of horizontal rolls with a single roll-pass opening adds materially to the overall expense which can be reduced by one-half by the use of horizontal rolls with two roll passes. While not so limited, the present invention overcomes the shortcomings and disadvantages in prior rolling practices by providing a method for rolling the rails and the like wherein horizontal rolls have two roll-pass openings, each for use at one of two different locations in the mill train. This permits the use of a pair of horizontal rolls as well as their chocks when desired, or even the entire mill stand at two different locations in the mill train. The rolling method of the present invention enhances the use of one or more spare mill stands in condition for readiness at some preselected remote location to the mill train for efficient replacement of a mill stand even though one of two roll-pass openings is worn or otherwise requires replacement.